The present invention relates to a communication method and apparatus, and particularly but not exclusively to a method and apparatus for providing data communication over a radio frequency channel in addition to voice, image or other data communication.
In a known radio frequency communication system, for example the INMARSAT-A (TM) Satellite Communication System, as described for example in xe2x80x9cSatellite Communications: Principles and Applicationsxe2x80x9d, Calcutt and Tetley, 1st edition 1994, users are connected to a public service telephone network (PSTN) through a satellite link to provide voice, facsimile and circuit switched data communication services. Attempts have been made to add a packet switched data service, in which users share a single channel, to the existing services available through INMARSAT-A. However, this requires the allocation of additional traffic channels, which adds to the cost of the system.
The document GB-A-2 232 562 describes a digital mobile telephone system with a data communications function. If a data communication is asymmetric, time slots which are not needed for communication in one direction are reassigned for data transmission in the opposite direction. The time slots may be reassigned to a different data terminal. However, such a system requires reallocation of part of a physical channel assigned for communication in one direction so as to change the direction and/or data terminal to which that part is assigned.
According to one aspect of the present invention there is provided a communication system in which a first station is arranged selectively to transmit a principal signal and a data signal. The first station establishes a communication channel to a second station and begins to transmit the principal signal to the second earth station. However, during a period in which no information can be transmitted to the second station, the first station transmits the data signal in such a way that it may be decoded by a third station and may also be received by the second station without affecting the information decoded by the second station.
During voice communication, a proportion of the time during which a channel is kept open is unused, for example when a user is listening without talking, so that the user""s transmit channel carries only noise. Likewise, in a facsimile communication, a terminal which receives facsimile data only transmits during the handshaking phases of the communication; at other times its transmit channel is unused.
In embodiments of the invention the available bandwidth of the communication channel is used with greater efficiency, and a data communication service may be provided concurrently with another service.
Preferably, the principal signal comprises a voice or facsimile signal.
Where the principle signal is a voice signal, the data signal is transmitted during periods of silence. The first earth station may transmit a silence code which is decoded by the second earth station to reproduce silence or low-level noise during a voice communication. Preferably, the third earth station interprets the silence code as a signal to receive data.
Alternatively, the first earth station may transmit a signal which is reproduced by the second earth station as acceptable (e.g. low-level) noise or silence but which is decoded by the third earth station as data, during the periods of silence.
Where the principal signal is a facsimile signal, the first station may detect whether the second earth station is transmitting facsimile data and therefore does not need to receive any facsimile data. The first station transmits the signal including the data signal in such a way that it does not activate the facsimile terminal at the second earth station, but activates the third earth station to receive data.
By the above measures, the second and third stations may receive data concurrently, with the second earth station only decoding the principal signal and the third earth station only decoding the data, but without interference between the signals. This aspect of the present invention extends in particular to the first station, the third station, apparatus within the first or third station which implement the essential features of the present invention and any method performed by such apparatus.
According to another aspect of the present invention, there is provided a method of encoding data not derived from a voice signal such that it appears to relate to a voice signal to a voice decoder but can be decoded to reproduce the data by a data decoder, and apparatus for performing said method.
According to another aspect of the present invention, there is provided a method in which a duplex channel assigned to a facsimile communication is shared so that, when a facsimile terminal is transmitting facsimile data in one direction, data is transmitted in the other direction for decoding by a data decoder in such a way that the facsimile terminal is not interrupted by the data.